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In this edition of The Watch, we talked with Simon Goldsworthy, Global Business Development Manager at our technology partner Wavefront Systems, about the latest advances in intruder detection technology.

The current conflict in Ukraine has highlighted the disruptive effect of drones. It’s shown that a rogue squadron of uncrewed systems can pose an asymmetric challenge to any harbour or ship. In the right hands, the damage can be significant. To date, airborne drones and uncrewed surface vessels have shown their ability to penetrate conventional defences. Though difficult to counter, they are anything but covert. Sinking below the water’s surface, the autonomous or unmanned underwater vehicle, takes advantage of the waters cloaking to pose a much more significant threat. Programmed from afar, these systems can follow a predetermined course and remain underwater for long periods, making them invisible to radars and optical systems. Though more complex to manufacture, their proliferation and availability over the last decade makes them a viable threat accessible to many near-peer adversaries. As of today, it is relatively simple to program a flight path and equip the drones with multiple payloads, each presenting a different threat.

Needle in a haystack

When detecting targets underwater, the technology of choice is sonar: sound waves are transmitted through the water and the reflections from targets can be used to detect and track them. Underwater drones, also known as autonomous underwater vehicle (AUV) systems or uncrewed underwater vehicle (UUV) systems, are challenging for sonar operators as they present a relatively small target. In a confined environment where the sound reflects from the seafloor, the sea surface or harbour walls and travels at different speeds through changing water column temperatures, the ability to detect these vehicles is no simple feat.

Our technology partners, Wavefront Systems, know how difficult this challenge is, they manufacture the world’s most deployed intruder detection sonar, Sentinel. This system was developed to detect and alarm when divers approach. Although divers are a challenging target, their speed, pattern of movement and equipment works against them. This weakness is exploited by Sentinel to keep customers safe across a myriad of environments whilst deployed from ships, on the side of a dam wall, on a seabed mounting or by expeditionary teams from RHIBs.

However, when Wavefront first looked at an AUV signature they realised a new challenge had emerged. AUV’s and UUV’s have much smaller noise signatures, can present a smaller target and travel at greater speed. Sentinel has always excelled at finding the needle in the haystack but with underwater drones, it was difficult to detect and then keep track of them at the ranges which we had become accustomed to when tracking divers.

Feel the noise

“Our R&D team are never happier than when they have a new challenge to defeat”, Goldsworthy recalls, “At the speed which AUV technology was being developed, we knew they were clearly going to become a threat to our customer base. Fortunately for us, AUV’s and UUV’s are machines with mechanical parts which is unfortunate for them, as we can listen out for their telltale systemic noise. We filed for a patent to enable us to combine Simultaneous In-band Active and Passive Sonar to track underwater targets, a technology we refer to as SInAPS®. That’s the story behind Sentinel 2. The results have been better than we ever hoped for.”

Early trials against one-person portable targets proved successful.

Merging active data returns, with the passive track provides a substantial performance improvement, especially so when active returns are weak and infrequent. We conducted early trials against one-person portable UUV’s which demonstrated the capability. Since that time, several trials have been conducted against different UUV’s of varying sizes and mechanical construction. Sentinel 2 can find more drones with small to medium vehicles commonly detected at distances of over 500 m. This means more time in which to plan your response.

Simple to operate

“Say you’d like to protect a vessel alongside, the Sentinel sonar head can be deployed on a tripod from the vessel itself, from a RIB or other small boat. The sonar system is lightweight and portable, weighing less than 35 kg. It can be connected to a topside portable computer system to run the user interface. In addition, sonar performance indicator (SPI) software allows the operator to take a local environment probe of the seawater to assess the expected perimeter of protection of the Sentinel wherever it is being operated.” Goldsworthy also said, “The Sentinel sonar system can also be deployed by cable, without a tripod. We provide a weighted plate that can be fitted to the bottom of the sonar to maximise its stability in the water. No calibration is needed. It can be turned on and used instantly, giving the operator an immediate view of their surroundings.”

Reporting to your chain of command

“Sentinel offers a user-friendly interface designed for non-sonar operators to use. It can be set up to show all detections within the sonar range or to show only the critical threats. The information is colour coded and an audible alarm is triggered for the end client. The system is automated and can output to leading command-and-control systems through simple application programming interface.” Goldsworthy adds,” Consider this: the sonar system generates thousands of signals from the sea floor, rocks, other vessels, pier legs and anything else in the underwater environment. Sentinel then uses active and passive detection algorithms to filter out the signals that are not from divers, subsea vehicles, or AUV’s. The remaining signals are then classified as critical or non-critical for the end user. With now fifteen years of experience delivering the world’s most deployed intruder detection sonar, we understand how important it is to report the correct data.”

Every ship can be equipped with the means to counter underwater drones today

“We have trained hundreds of users and are able to accommodate different levels of ability. From maintenance to operations, we can cater for multiple trainees and all we want from your team is a handful of days. We can even train your trainers if required.”

Please contact Forcys to find out more.

Executive Summary

Naval warships are vulnerable alongside. While gangway staff are present 24/7 at a ship’s entry point and bases are patrolled with security guards, cameras and fencing, the fact remains that today one of the greatest asymmetrical threats can come from a perpetrator, with basic diving equipment and little training, approaching a ship underwater, undetected and with relative ease.
This paper by Sean Leydon, Regional Manager at Asia Pacific for Forcys, discusses the issues that face modern warships and submarines alongside either at home or away, the degrees to which they have been targeted under this vulnerable state, and solutions to provide both the warship and its crew, protection.

Asymmetric Threats

History has recorded several examples of what the asymmetrical threat of placing rudimentary explosive devices on the hulls of ships can cause. From the Vietnam conflict where a diver sank the 9,000-ton USS Card with an explosive attached to the hull, to the more recent incident on 12th May 2019 where four commercial ships were targeted in the port of Fujairah in what the UAE described as a “sabotage attack”. An international investigation concluded that there were “strong indications” that the attacks were perpetrated by a sophisticated “state actor” that breached their hulls with explosives. The threat is real and so far, has proven difficult to detect and counteract without a dedicated intruder detection sonar.

From Surface Attack to Underwater Attack – The Increased Difficulty

While surface attacks on vessels in port have delivered devastating effects, the barrier to transition this threat to the more complex underwater domain has been mainly due to the access and cost of the technology and the training required to utilise it.
From the early 1990’s however, the number of sport diving rebreather brands and underwater motorised equipment has rapidly increased. Advanced sport divers increasingly tackle longer, deeper, riskier dives using equipment once available only to armed forces or professionals. Acquiring rebreather or scuba equipment and receiving technical training is no longer a barrier.

Protection of Assets – Capability Phases

Security of warships in port includes the requirement to inspect, detect and identify anomalies on ships, and wharfs and to also employ fast boats as a deterrent. As previously mentioned, base security and surveillance in ports usually covers the above water threat, including those on the water such as boats and swimmers, however, it does not cover the underwater threat from divers and underwater vehicles.
The following discusses the different phases required to defend against the underwater threat.

Phase One – Surveillance

The system must be capable of providing a constant picture of the underwater environment and any changes to it. The capability used must not only be able to provide surveillance, but be transportable, scalable, networked, and simple to operate. While permanent systems can be setup in a ship’s home port, it must be transportable and easy to setup by ship’s company if the ship has berthed in a foreign port.

Phase Two – Detection

Surveillance is only the first step of defence against a threat. The system in place must also be able to detect when an intruder (not marine life or false targets) is within an area of interest. The system needs to continuously monitor the surrounding area, understand the difference between an intruder or false target, and track the designated target of interest that it has acquired.

Phase Three – Tracking

Sonar is capable of generating thousands of detections. Each one of these needs to be tracked. Their direction of travel and behaviour will provide important cues to the next phase.

Phase Four – Classification

Whether the intrusion is from a scuba diver, rebreather or underwater vehicle, the system should provide the classification of the target for the end user. This will allow the response process to be coordinated with the appropriate action.

Sentinel: The world’s leading Intruder Detection Sonar

The Sentinel Intruder Detection Sonar (IDS) from Wavefront Systems is a commercial off-the-shelf (COTS), compact, lightweight system with a proven track record of global performance for the military and private use. Using sophisticated sonar processing algorithms, Sentinel provides fully automatic detection and tracking of potential underwater threats and targets of interest while analysing their behaviours. It provides protection against underwater threats including:


Weighing only 32 kgs, the Sentinel head is ideally suited for deployed warships or security units responsible for its safe arrival/berthing. Consisting of a sonar head, workstation, cables, and deployment frame, Sentinel is mobile and simple to set up. This enables operators to quickly initialise the system and secure a berth, whether deployed over the side or through the ship’s hull.


Operation of the Sentinel IDS GUI is simple, intuitive, and highly configurable. The tactical picture can be fused with localised charts and tailored to operators’ requirements, allowing the effective detection, tracking and classifying of targets.

Sentinel operations

Classification – Active and Passive Tracking

Sonars can receive signals either passively (receive only) or actively (transmit and receive) – Sentinel, uniquely combines both active and passive processing using it’s patented SInAPS® technology (Simultaneous In-band Active and Passive Sonar) allowing the passive emissions from the target to help detection and tracking when active tracking may not be possible. Furthermore, this can also help with classification of targets (for example marine life do not generally emit sound, while an AUV provides constant propeller noise).
Both types are tracked and displayed in an intuitive way which helps the operator associate the tracks and make the correct tactical decisions.

Full Aperture Zoom classification of rebreather (left), AUV (centre) and scuba (right)


Sentinel is designed to protect ports, harbours, commercial and naval vessels, offshore platforms, and waterside facilities. Over 200 systems sold and installed, Sentinel operates across all oceans and is already used to protect naval bases, military vessels and critical national infrastructure.

Networked System

Sentinel is capable of standalone operation or in multiple sonar head configuration for a larger security perimeter. The Sentinel sonars heads can be deployed at points around a port and work together using Wavefronts unique Super Inheritance™ technique to deliver a single set of threats to the user.

Networked Sentinel sonars displayed working together

In Summary

Wavefront Systems deployed the first Sentinel in 2009, it was a revolutionary sonar in that it was both extremely portable and offered unrivalled range and performance. Since that time the system has evolved to enable new methods of deployment, networked capabilities and improved tracking. The recent addition of simultaneous passive sonar capability means that Sentinel is yet again setting the performance at a higher bar than was thought possible. This is even more important as the threat from state actors is on the increase

If you would like to learn more about Sentinel IDS please feel free to get in touch with the team.